The overall objective of this proposal is to understand the relationship between the synthesis of the organic biomacromolecules and the formation of the inorganic phase of the skeletal matrix. Our hypothesis is that calcium mineral plays a dual functional role during normal embryonic skeletogenesis. We postulate that, in addition to directly forming the mineral phase of the skeletal matrix, calcium also regulates the sequential cellular differentiation and the synthesis of organic matrix components by the various cell types of the skeletal element. To test this hypothesis, we propose projects which will utilize an experimental system of calcium-deficient embryonic growth consisting of chick embryos maintained in long-term culture without the eggshell, its major calcium source. This system offers the advantage of permitting experimental modulation of calcium availability to the developing embryonic skeleton. The expression and synthesis of a differentiation-specific biomacromolecule, collagen, will be studied in the skeletal tissues of these embryos with particular attention to collagen genetic type transition which will be correlated to the status of calcium accumulation and skeletal mineralization in the developing embryo.